The study aims to delineate the extent to which colonization of S. mutans, S. sanguis and organisms of the lactobacillus species precede the clinical manifestation of fissure caries. It will also examine the quantitative relationship between these organisms, over time, in both clinically sound and in clinically carious teeth. The time sequence of colonization of the organisms, both in absolute terms and relative to each other will be be related to the development, or absence of development, of clinical caries, and the study will seek to demonstrate microbiological developments that predict the diagnosis of clinical caries with a high degree of probability. In addition, the study will seek to validate a simple predictive test for dental caries, suitable for use in the dental office. This test is based on the ratio of S. mutans to S. Sanguis (MS ratio) in the plaque sample. This three year longitudinal study is an epidemiological-microbiological collaboration. There were 503 children in the nonfluoridated community of Coldwater, Michigan, who began the study in April, 1979. Each child will have individual plaque samples taken from the occlusal fissures of initially caries-free mandibular first molars at six-month intervals, and will receive a clinical dental examination each year. Part of each plaque sample will be plated out, incubated, and a count made of colony-forming units resembling S. mutans, S. sanguis, and lactobaccilli. For the predictive test, another part of the sample will be incubated in a mannitol-arginine broth, which changes color above a certain MS ratio. Results of the bacterial counts will be compared with the carious status of the teeth as determined by the clinical examinations, and results from different examination and sampling cycles compared. Results of the color-change test can then be validated against the bacterial counts.